FILE: Jan. 24, 2013: Then-Sen. John Kerry, D-Mass., sits before a Senate committee during his confirmation hearing to be secretary of State.
(AP)

Aboard Secretary of State John Kerry's plane to London – As Secretary of State John Kerry winged his way across the Atlantic on the first leg of his first overseas trip as America's top diplomat, his aides were scrambling on several continents to salvage one of the trip's most critical and eagerly-awaited sessions.

Kerry was slated to visit four European capitals -- London, Berlin, Paris and Rome -- before a Mideast swing that will take him to Turkey, Egypt, Saudi Arabia, the United Arab Emirates and Qatar.

During the visit to Italy, the State Department had announced, Kerry would have the chance to meet with leaders of the Syrian opposition that have battled the regime of Syrian President Bashar al-Assad for two years, in a civil war that has claimed roughly 70,000 lives.

Over the weekend, however, the Syrian Opposition Council -- the main umbrella group for the broad array of rebel politicians, clerics and soldiers, many of them Islamists with ties to al-Qaeda and other unsavory networks -- announced it was boycotting the session with Kerry in Rome.

The Syrians complained that the United States -- which has blacklisted al-Nusra, the al-Qaeda-linked group that has emerged as one of the most effective combat brigades in the battle against Assad's military -- has not been even-handed enough in its treatment of the rebels.

The Obama administration has provided millions in non-lethal aid to the rebel forces but has balked at supplying arms to them, citing fears that the weapons would wind up in the hands of anti-American Islamists.

Senior U.S. officials said Sunday that frenetic efforts were underway to persuade the Syrian rebels to rethink their position and agree to a meeting with Kerry before he reaches Italy on Thursday.

Robert Ford, the American ambassador to Syria who has not visited Damascus since the U.S. shuttered its embassy, was dispatched to Cairo to work on the Syrians from there, while acting Assistant Secretary of State Beth Jones, head of the Department's Near Eastern bureau, was also working the phones and was expected to meet up with Ford and Kerry in Rome.

Additionally, the U.S. is relying on third-party countries to exercise suasion on the recalcitrant rebel leaders.

"The Syrian opposition leadership is under severe pressure now -- from its membership, from the Syrian people -- to get new support from the international community," said a senior Obama administration official traveling with Kerry to London early Sunday. "There's quite a bit of internal discussion about the value of going to international conferences."

American diplomats described the internal disputes amongst the rebels as a messy business in which many of those on the front lines -- the fighters and commanders who are starving, bleeding and dying -- view with increasing suspicion the rebels' civilian leaders who spend more time meeting with foreign diplomats in fine European hotels.

At the same time, some of those rebel figures who have been most vocal in rejecting the meeting with Kerry are said in private to favor it and to be asking Washington to do more to provide them with the political cover they feel would enable them to make the trip.

To meet that last request, the State Department late Saturday night issued a statement condemning the Assad regime's latest assaults on the devastated city of Aleppo, and taking note of the rebels' recent advances in and around the capital.

"We will make our case," the American official said, casting the Rome session as an opportunity for the rebels "to meet our new secretary of state, to speak directly to him."

During a meeting in Washington last week with Jordan's foreign minister, Kerry said his upcoming trip would provide him with a chance to develop some new ways of changing the "calculation about where things are" that President Assad presently maintains.

Were his planned meeting with the Syrian rebels to be scuttled, it would mark a setback in the new secretary's nascent effort at changing Assad's thinking and a minor embarrassment on his first overseas trip.

Kerry is said not to be bothered too greatly by the uncertainty hovering over the Rome stop and to believe that the meetings will ultimately go forward as announced.

That sense of equanimity was already on display in the session with the Jordanians last week.

"I'm an optimist," Kerry told Foreign Minister Nasser Judah on February 13. "If I weren't an optimist, I wouldn't have taken this job."